I want to modify the doorway to a 92-inch header. How would I do this?
If 92 inches is shorter than your existing vertical measurement of the header position within the door frame opening, then you could simply add the required amount of material to the bottom of your existing header to fill the gap. If 92 inches is taller than your existing vertical measurement of the header position within the door frame opening, then you will have rebuild the majority of your door frame or purchase a new one altogether. If you rebuild the door frame, you will need to remove the nails or screws holding the jack studs in place. Remove the studs, cut new ones to the right length, raise your header, install and fasten the new jack studs, and reposition your header.
30" or so is typical. Make sure to leave enough room for any furniture or appliances that need to go in or out. 32-36" is helpful if you have enough room in the rough opening.